HR: 11:05h
AN: U42A-04    [Abstracts]
TI: Age of the Xalnene Ash, Central Mexico and Archeological Implications
AU: * Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: * Renne, P R
EM: prenne@bgc.org
AF: Dept. of Earth and Planetary Science, Univ. of California, Berkeley, CA 94720 United States
AU: Feinberg, J M
EM: feinberg@eps.berkeley.edu
AF: Dept. of Earth and Planetary Science, Univ. of California, Berkeley, CA 94720 United States
AU: Waters, M R
EM: mwaters@tamu.edu
AF: Depts. of Anthropology, Geography, and Center for the Study of the First Americans, Texas A&M Univ., College Station, TX 77843 United States
AU: Cabrales, J A
EM: arromatu5@yahoo.com.mx
AF: Laboratorio de Arqueozoologia, Subdireccion de Laboratorios y Apoyo Academico, Instituto Nacional de Antropologia e Historia, Mexico, DF 06060 Mexico
AU: Castillo, P O
EM: patricia_ochoa_cast@yahoo.com
AF: Subdireccion de Arqueologia, Museo Nacional de Antropologia, Mexico, DF 11560 Mexico
AU: Campa, M P
EM: mapcampa@yahoo.com
AF: Proyecto Cuicuilco, Instituto Nacional de Antropologia e Historia, Mexico, DF 06700 Mexico
AU: Knight, K B
EM: kbk@berkeley.edu
AF: Dept. of Earth and Planetary Science, Univ. of California, Berkeley, CA 94720 United States
AB: Human footprints ~40 ka old have been reported from the Toloquilla quarry near Valsequillo Reservoir, ca. 15 km south of the city of Puebla in central Mexico (http://www.mexicanfootprints.co.uk/default.htm). If correct, this would be important evidence for early peopling of the Americas. The indentations interpreted as footprints and other ichnofossils occur on the surface of an indurated basaltic lapilli tuff within a several meter thick sequence of thinly bedded (1-10 cm) tuffs of similar character, lacking paleosols, erosional features or interlayered sediments, informally known as the Xalnene ash. A sample was collected at 18°55.402` N latitude and 098°09.375` W longitude from the surface on which the purported footprints occur. Lapilli were separated and analyzed by incremental heating ⁴⁰Ar/³⁹Ar methods, yielding 9 indistinguishable plateau ages averaging 1.30 ±0.03 Ma (2σ) for single lapilli (N=6) and multiple lapilli (N=3) subsamples. Though some minor discordance (presumably due to ³⁹Ar recoil) is manifest in 5 of the age spectra, all plateaux comprise >60% of the ³⁹Ar released and 4 or more consecutive steps. Paleomagnetic data from azimuthally unoriented bulk samples of 11.25 cm³ reveal a reverse polarity (I = -32.1°) thermoremanent component carried by titanomagnetite and a normal polarity component carried by goethite. Measurements on individual matrix-free lapilli lack the goethite component, which is presumed to be associated with the clay-rich cement. Consistency of the reverse component implies deposition of the lapilli at supra-Curie temperatures, with no postdepositional reworking. Reverse polarity is consistent with deposition during chron C1r.2r (1.77 to 1.07 Ma) as indicated by the ⁴⁰Ar/³⁹Ar data. If the features observed on the tuff are indeed footprints, their 1.3 Ma antiquity would be truly remarkable, predating by far any other evidence for human presence in the Americas and in fact predating the evolutionary emergence of Homo sapiens (in Africa) by more than 1 Ma. We conclude that the identification of these features as syn-depositional human footprints is likely erroneous.
DE: 0444 Evolutionary geobiology
DE: 1040 Radiogenic isotope geochemistry
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 1527 Paleomagnetism applied to geologic processes
SC: Union [U]
MN: Fall Meeting 2005